Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings are particularly useful as topcoats for which exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels.
Single-layer topcoats and the clearcoats of color-plus-clear composite coatings, however, require an extremely high degree of clarity and gloss to achieve the desired visual effect. Such coatings also require a low degree of visual aberrations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI). As such, these coatings are especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out. It is often difficult to predict the degree of resistance to environmental etch that a high gloss topcoat or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as known high-solids enamels, do not provide the desired level of resistance to environmental etch when used in high gloss coatings such as the clearcoat of a color-plus-clear composite coating.
Various compositions have been proposed to meet the above requirements for use as the topcoat coating or as the clearcoat of a color-plus-clear composite coating, including polyurethanes, acid-epoxy systems and the like. However, many prior art systems suffer from disadvantages such as coatability problems, marginal compatibility with the pigmented basecoat, solubility problems, and marginal appearance. Moreover, while one-pack compositions are preferred to two-pack compositions (in which the reactive component must be separated before application to prevent premature reaction), very few one-pack coating compositions have been found that provide satisfactory resistance to environmental etch, especially in the demanding environment of automotive coatings.
There is also a continuing desire to reduce the volatile organic content (VOC) of coating compositions. Previous low-VOC coating compositions generally have had less environmental etch resistance. Reducing the VOC must be done without sacrificing the rheological properties of the coating composition required for trouble-free application of the composition while maintaining the desired level of appearance. In addition, it is desirable to provide coatings with a good combination of properties such as durability, hardness, flexibility, and resistance to scratching, marring, solvents, and acids.
Curable coating compositions utilizing carbamate-functional resins are described, for example, in U.S. Pat. Nos. 5,693,724, 5,693,723, 5,639,828, 5,512,639, 5,508,379, 5,451,656, 5,356,669, 5,336,566, and 5,532,061, and U.S. application Ser. Nos. 08/886,321, filed Jul. 1, 1997, 08/698,529, filed Aug. 15, 1996, 08/719,670, filed Sep. 25, 1996, 08/166,277, filed Dec. 13, 1993, 08/339,999, filed Nov. 15, 1994, 08/333,917, filed Nov. 3, 1994, 08/176,608, filed Jan. 3, 1994, 08/287,351, filed Aug. 8, 1994, 08/804,239, filed Feb. 20, 1997, 08/333,804, filed Nov. 3, 1994, 08/884,613, filed Jun. 30, 1997, 08/885,638, filed Jun. 30, 1997, 08/513,587, filed Aug. 10, 1995, 08/867,547, filed Jun. 2, 1997, 08/547,514, filed Oct. 24, 1994, 08/547,513, filed Oct. 24, 1994, 08/547,174, filed Oct. 24, 1994, 08/698,524, filed Aug. 15, 1996, 08/698,526, filed Aug. 15, 1996, 08/667,261, filed Jun. 20, 1996, 08/698,528, filed Aug. 15, 1996, 08/698,522, filed Aug. 15, 1996, 08/698,572, filed Aug. 15, 1996, 08/698,523, filed Aug. 15, 1996, 08/673,935, filed Jul. 1, 1996, 08/886,321, filed Jul. 1, 1997, and 08/831,810, filed Apr. 2, 1997, each of which is incorporated herein by reference. These coating compositions can provide significant etch advantages over other coating compositions, such as hydroxy-functional acrylic/melamine coating compositions. It may often be desirable, however, to provide still further improvements in the above-described coating properties. In particular, it has been discovered that coating compositions having improved properties are obtained by blending carbamate- or urea-functional resins with resins having carbamate, urea, and/or different functionalities or bringing different properties to the cured coating formed from the combination. The blends of the present invention provide coatings that have improved environmental etch resistance and improved exterior durability (weathering or exposure durability), as well as lower volatile organic content (VOC).